1. Field of the Invention
The present invention relates to holographic optical processing apparatus and methods, and particularly to those apparatus and methods which overlay holograms in a common optical storage medium.
2. Discussion of the Prior Art
The so-called volume storage of holograms in electro-optic materials, such as bismuth silicon oxide, results in the overlaying of one hologram by another hologram. Such overlaying can occur by changing the angle of incidence of incoming interfering light beams, wave lengths of operation or physical location of the beams. Such overlaying tends to reduce the average intensity of all of the holograms in the overlaid area. Additionally, light-interfering contaminants (dust, etc.) or variations in the storage elements can change the intensity of the holograms. As a result, reproduction of a stored hologram results in variations in the intensity of the reproduced pixels (picture elements) of the hologram. Accordingly, it is desired to control the operation of the apparatus such that, even with variations in intensities caused by overlaying and other factors which can cause perturbations in holographic intensities, the intensities will be normalized, such as to an average intensity value of the hologram.
Holograms are generally processed using arrays of photoelements, either to emit light beams which interfere with other emitted light beams to create a hologram, or to intercept the hologram for reconstructing the image stored in the hologram. Some rectangular arrays (such as in video applications) of photoelements have included a small number of photoelements used to indicate "washout" of an image being processed. Identification of such washouts of images does not accommodate variations in individual pixel intensity levels in an array of pixels being processed. Such accommodation becomes important when data is stored in the hologram as one level of light intensity out of a plurality of possible light intensity levels; for example 16 levels of light intensity. Such light intensity levels are considered as light states. Accordingly, it is desired to provide a normalization of the intensity of a hologram for ensuring a more faithful reproduction of stored images.